Bleaching of pulp for papermaking is roughly divided into bleaching with a peroxide, bleaching with a chlorine-containing oxidizing agent, and bleaching with a reducing agent, and these have been employed depending on the kind of pulp and purpose of use in consideration of their bleaching characteristics and effects.
The bleaching with a peroxide represented by H.sub.2 O.sub.2 is employed mainly for bleaching of mechanical pulp and for deinking/bleaching of waste newspaper because of its bleaching effect on pulp containing lignin.
Since the bleaching with a chlorine-containing oxidizing agent is effective for chemical pulp containing a slight amount of lignin, bleaching with Cl.sub.2, NaClO or ClO.sub.2 is mainly carried out in multistage bleaching of kraft pulp, and bleaching with NaClO in bleaching of waste wood-free or mechanical paper. For example, in the bleaching of waste wood-free or mechanical paper, a 12% NaClO solution is added to pulp in an amount of about 8% based on the amount of pulp and mixed therewith, whereby the bleaching is carried out.
The bleaching with a reducing agent is excellent in decolonization of dye type coloring materials, but it has a limited-bleaching capability for peroxide type chemicals and chlorine-containing chemicals and entails a high cost. For these reasons and the like, it has been employed mainly for bleaching of mechanical pulp demanding a low brightness grade to be incorporated into newspaper, post-bleaching after H.sub.2 O.sub.2 bleaching of mechanical pulp demanding a high brightness grade and post-bleaching after H.sub.2 O.sub.2 deinking/bleaching of waste newspaper. In general, Na.sub.2 S.sub.2 O.sub.4 or TUDO of the dithionite family has been used as the reducing agent. Of these, Na.sub.2 S.sub.2 O.sub.4 has been mainly used from the viewpoint of bleaching capability, chemicals cost, etc. On the other hand, TUDO is a dithionite type reducing agent like Na.sub.2 S.sub.2 O.sub.4, and has a bleaching effect equal to that of Na.sub.2 S.sub.2 O.sub.4, but it has not been often used because it entails a high chemicals cost.
However, TUDO bleaching has recently come to be spotlighted as a substitute for NaClO bleaching in bleaching of waste wood-free or mechanical paper and as a substitute for Na.sub.2 S.sub.2 O.sub.4 bleaching in post-bleaching after H.sub.2 O.sub.2 deinking/bleaching of waste newspaper, from the viewpoint of the problem of waste water pollution by organic chlorine compounds represented by dioxins, demand for energy conservation, decoloring effect on dye type coloring materials, etc.
By the way, NaClO bleaching is not expensive and has an excellent decoloring capability for dye type coloring materials, and hence in bleaching of waste wood-free or mechanical paper, NaClO bleaching has heretofore been carried out by a process roughly co represented by the following:
Pulper!.fwdarw.Soaking/deinking!.fwdarw.Washing!.fwdarw.Kneading!.fwdar w.NaClO bleaching!.fwdarw.Washing!.fwdarw.Papermaking steps!
However, because of the problem of production of organic chlorine compounds as by-products by employment of chlorine-containing chemicals, there has been a desire for development of a bleaching method as a substitute for bleaching with NaClO for the purpose of avoiding the production of the by-products. As substitute methods for NaClO bleaching, H.sub.2 O.sub.2 bleaching, Na.sub.2 S.sub.2 O.sub.4 bleaching, TUDO bleaching, etc. have been investigated. Of these, TUDO bleaching has come to be considered hopeful as a substitute for NaClO bleaching for the following reasons.
First, H.sub.2 O.sub.2 bleaching has a sufficient bleaching effect on natural pigments by nature but has an insufficient decoloring effect on dye type coloring materials from leaflets and colored wood-free paper among waste wood-free or mechanical papers. Therefore, it is not sufficient as a substitute for NaClO bleaching.
Na.sub.2 S.sub.2 O.sub.4 bleaching has a sufficient decoloring effect on dye type coloring materials by nature but is disadvantageous in that it is subject to oxidative decomposition by oxygen in air. Therefore, it requires a special bleaching apparatus which excludes air. When a presently used NaClO bleaching apparatus is diverted as it is, no satisfactory bleaching can be carried out owing to the influence of oxygen in air. Therefore, Na.sub.2 S.sub.2 O.sub.4 bleaching is not suitable as a substitute for the NaClO bleaching. In detail, in the presently used NaClO bleaching equipment bleaching conditions are carried out at a state of pulp having a relatively high consistency of PC 10 to 30%. Accordingly, equipment from a chemicals-mixer to a bleaching tower are substantially located in a system open to air, and oxygen in air inhibits the bleaching effect of Na.sub.2 S.sub.2 O.sub.4.
On the other hand, TUDO bleaching has a considerable decoloring effect on dye type coloring materials, like Na.sub.2 S.sub.2 O.sub.4 bleaching, and TUDO has a property of reacting with oxygen in air slowly as compared with Na.sub.2 S.sub.2 O.sub.4. Even when a presently used NaClO bleaching equipment is diverted as it is, a loss due to oxidative decomposition is relatively small and a bleach brightness equal to that attained by NaClO bleaching can be attained by choosing a suitable using amount of TUDO.
As post-bleaching after H.sub.2 O.sub.2 deinking/bleaching of waste newspaper, Na.sub.2 S.sub.2 O.sub.4 bleaching has been mainly employed from the viewpoint of chemicals cost. Conventional bleaching with Na.sub.2 S.sub.2 O.sub.4 is carried out under conditions of a low pulp consistency of PC 3 to 5% and a temperature of 50-70.degree. C. Since a large amount of thermal energy is required for adjusting the temperature to 50-70.degree. C. at a low pulp consistency, bleaching at a state of pulp having a high consistency of PC 10 to 30% has recently come to be aimed at for energy conservation. In the bleaching at a state of pulp having a high consistency, contamination with air tends to occur as in the case of the aforesaid waste wood-free or mechanical paper, and hence there is a problem that no satisfactory effect can be obtained by Na.sub.2 S.sub.2 O.sub.4 bleaching. For exhibition of the bleaching effect of Na.sub.2 S.sub.2 O.sub.4, it is necessary to use a bleaching equipment capable of excluding air for preventing oxidative decomposition by air. In this case, a considerable equipment investment is necessary.
On the other hand, since TUDO undergoes relatively rarely oxidative decomposition by oxygen in air. TUDO bleaching is suitable for bleaching pulp having a high consistency by a usual equipment while achieving a saving in energy.
Since TUDO is characterized in that it undergoes oxidative decomposition by oxygen in air relatively rarely as described above, it has come to be considered hopeful as a substitute bleaching method for all NaClO bleaching of deinking pulp from waste wood-free or mechanical paper and Na.sub.2 S.sub.2 O.sub.4 bleaching after H.sub.2 O.sub.2 deinking/bleaching of waste newspaper.
However, TUDO bleaching, on the other hand, has the following defects. Current TUDO bleaching is carried out using powdery TUDO manufactured as product by a maker, and the cost of this bleaching treatment is high for the following reasons. For one thing, the TUDO powder is obtained by reacting a thiourea solution with H.sub.2 O.sub.2 fluid and purifying the resulting TUDO fluid, followed by concentration and drying. In this case, expensive equipment for the reaction, purification, concentration and drying are necessary, and a large amount of energy is required for controlling the reaction and for the purification, concentration and drying. Moreover, a part of TUDO is decomposed and lost in the procedures of reaction and commercialization, and consequently the production cost of the chemical is very high in itself. Another problem is that when the powdery TUDO is dissolved at a bleaching site and added to pulp, a loss of its active ingredient by decomposition is caused by the influence of oxygen in air during the dissolution with stirring and the mixing with the pulp, so that the bleaching effect is lessened as much.
Thus, the treatment cost of TUDO bleaching is high from using TUDO product of high cost in prior art. In addition, during the transport, storage and use of a TUDO product, its loss is caused by gradual oxidative decomposition by oxygen in air. Therefore, in the bleaching industry, there is desired a TUDO bleaching process which is free from the above defects and is easy to apply and practice on the site.